Mockito with newInstance method - mockito

Question

I have a class-under-test that has the following code :
public void getDetails (String message){
DocumentBuilderFactory docBuilderFactory = DocumentBuilderFactory.newInstance();
DocumentBuilder docBuilder = docBuilderFactory.newDocumentBuilder();
StringReader strReader = new StringReader(message);
InputSource inputSrc = new InputSource(strReader);
Document doc = docBuilder.parse(inputSrc);
...
}
I want to write a JUnit for this piece of code using Mockito.
I tried various things like :
DocumentBuilderFactory docBuilderFactoryMock = Mockito.mock(DocumentBuilderFactory.class);
Mockito.when(DocumentBuilderFactory.newInstance()).thenReturn(docBuilderFactoryMock);
But I get the Exception:
org.mockito.exceptions.misusing.WrongTypeOfReturnValue:
DocumentBuilderFactory$$EnhancerByMockitoWithCGLIB$$23223735 cannot be returned by toString()
toString() should return String
If you're unsure why you're getting above error read on.
Due to the nature of the syntax above problem might occur because:
1. This exception *might* occur in wrongly written multi-threaded tests.
Please refer to Mockito FAQ on limitations of concurrency testing.
2. A spy is stubbed using when(spy.foo()).then() syntax. It is safer to stub spies -
- with doReturn|Throw() family of methods. More in javadocs for Mockito.spy() method.
If I do the following:
DocumentBuilderFactory docBuilderFactoryMock = Mockito.spy(DocumentBuilderFactory.newInstance());
DocumentBuilder documentBuilderMock = Mockito.mock(DocumentBuilder.class);
Mockito.when(docBuilderFactory.newDocumentBuilder()).thenReturn(documentBuilderMock);
docBuilderdocBuilderMockito.when(docBuilderFactoryMock.newDocumentBuilder()).thenReturn(docBuilderFactoryMock);
and debug my code then I see that the class-under-test does not use my Mock objects anywhere but creates its own objects and throws a SAXParseException at
Document doc = docBuilder.parse(inputSrc);

Answer 1

Unit testing is intended to test your components/classes, not the library components/classes that are used.
Your class is parsing a String as xml content and to test that, you will find that providing a set of xml Strings with known output is the best way to test.
Simply pass a known xml String to your class under test and assert that the resulting model that is parsed, contains the data that you expect for that xml content.
I don't think that you need any mocking.